1. Field of the Invention
The present invention relates to a high-density magnetic recording technology and, more particularly, to a method of patterning magnetic products, magnetic recording media such as patterned media produced by this method, and other magnetic products, and to a magnetic recording apparatus equipped with such magnetic recording media.
2. Description of the Related Art
In recent years, because of an increase in a surface recording density of a magnetic recording medium along with an increased recording capacity of a hard disk drive (referred to as HDD, hereinafter), each recording bit size on the magnetic recording medium has become extremely minute, about several 10 nm. To obtain a reproducing output from such a minute recording bit, saturation magnetization and a film thickness as large as possible must be secured for each bit. However, the minute recording bit reduces a quantity of magnetization per bit, and there arises a problem that is loss of magnetization information due to magnetization reversal by “thermal fluctuation”.
Generally, it is said that this “thermal fluctuation” has a larger effect as a value of Ku·V/kT (Ku: a magnetic anisotropy constant, V: a minimum unit volume of magnetization, k: Boltzman's constant, T: an absolute temperature) is smaller, and it is empirically said that magnetization reversal occurs because of “thermal fluctuation” at Ku·V/kT<100.
In the case of a magnetic recording medium of a longitudinal magnetic recording mode, since a demagnetization field becomes strong in a recording bit of a high recording density region, the medium tends to be affected by “thermal fluctuation” even while a magnetic particle size is relatively large. On the other hand, in the case of a magnetic recording medium of a perpendicular magnetic recording mode, since growth of a magnetic particle in a film thickness direction enlarges a minimum unit volume of magnetization V while keeping a particle size of a medium surface small, the effect of “thermal fluctuation” can be suppressed. However, if a density of the HDD is increased much more in the future, there may be a limit to resistance to thermal fluctuation even for the perpendicular magnetic recording mode.
As a method for solving the problem of the thermal fluctuation resistance, a magnetic recording medium called “patterned medium” attracts attention. The patterned medium generally means a magnetic recording medium in which a plurality of magnetic material regions to be recording bit units are independently formed in a nonmagnetic material layer. In a general patterned medium, for the nonmagnetic material layer, for example, an oxide such as SiO2, Al2O3 or TiO2, a nitride such as Si3N4, AlN or TiN, carbide such as TiC, and boride such as BN are used, and ferromagnetic material regions are selectively formed in these nonmagnetic material layers.
In the patterned medium, since the ferromagnetic material regions which are the recording bit units are independent of each other, interference between the respective recording bits can be prevented. Therefore, the patterned medium is advantageous for reducing recording loss and noises which are caused by adjacent bits. Moreover, patterning increases resistance of domain wall movement (pinning effect of domain wall), making it possible to improve magnetic properties.
On the other hand, in the case of the HDD, positioning of a magnetic head in a target position (target track) on the magnetic recording medium or moving speed is controlled based on servo information pre-recorded on the magnetic recording medium. Generally, the servo information is recorded in each of servo regions (servo sectors) radially provided at predetermined intervals in a circumferential direction on the magnetic recording medium.
Normally, writing of the servo information is carried out by using a servo writing device called a servo track writer. After assembling the magnetic recording medium and the magnetic head into a casing of a HDD main body, the servo information is written. However, as a recording density of the HDD becomes much higher, the quantity of the servo information is increased proportionately thereto. Then, an area of the servo region on the magnetic recording medium is consequently increased, reducing an area of an effective recording region (data region) in contradiction.
On the other hand, studies have recently been conducted on a magnetic recording medium structure of a “deep layer servo system” which has a servo region buried in a deep layer different from a magnetic recording layer. In this structure, since the recording region and the servo region can be formed by being laid on each other, a full surface of the magnetic recording medium can be used as the recording region, and the servo region can also be formed on the full surface of the magnetic recording medium. Thus, without sacrificing the recording region, the magnetic head is enabled to perform highly accurate tracking at any point on the disk.
To fabricate the above-described patterned medium, it is necessary to form fine magnetic material patterns in a large area. On the other hand, a magnetic random access memory (MRAM) has recently attracted attention as a new nonvolatile memory element. Manufacturing of the MRAM also necessitates highly integrated magnetic material patterning.
Conventionally, for such magnetic material patterning, the following four processes have mainly been employed: first, a process for forming a magnetic material thin film to be fabricated; second, a photolithography process for forming a resist film on the magnetic material thin film, and for forming a pattern on the resist film by using photon energy, electron beams, ion beams or the like; third, a process for etching the magnetic material thin film using the resist pattern as a mask; and fourth, a process for removing remaining resists or residuals left after the etching. Among the above processes, the thin film formation process, the photolithography process, and the residual removal process can use methods applied in semiconductor processes. However, since the magnetic material is hard to be etched unlike a general semiconductor material, it is difficult to use normal reactive ion etching (RIE) used in a semiconductor process. Instead, therefore, a physical etching method such as ion milling, in which field-accelerated ions sputter on a sample surface, has been used.
FIGS. 1A to 1E show a conventional method for manufacturing a patterned medium using ion beam milling. That is, as shown in FIG. 1A, a ferromagnetic material layer 520 containing Fe, Co, Ni or the like is first formed on a substrate 510 of Si or the like by using a sputtering method or the like. Then, on this ferromagnetic material layer 520, a resist pattern 530 corresponding to a desired pattern is formed by electron beam writing. Further, as shown in FIG. 1B, ion beam milling is carried out by using this resist pattern 530 as a mask, and an exposed portion of the ferromagnetic material layer 520 is subjected to etching. Then, as shown in FIG. 1C, a remaining resist film is removed. As shown in FIG. 1D, a nonmagnetic material layer 540 is coated to fill grooves formed by the ion milling. Lastly, by subjecting the substrate surface to chemical mechanical polishing (CMP), a patterned medium shown in FIG. 1E is obtained.
However, in the above-described conventional manufacturing method, since the ferromagnetic material layer 520 is fabricated using the ion beam milling, damage remains on a crystal structure of the fabricated surface. Thus, fabrication with no damage is desired to further improve magnetic properties.
In addition, as the etching by the ion milling is physical, there is almost no difference in etching rates due to a difference between materials to be etched. As the ferromagnetic material layer 520 and the resist pattern 530 are scraped at approximately the same rate, an aspect ratio of a shape that can be fabricated depends on a thickness of the resist pattern 530 as a mask. If there is about 20 nm difference in level between a surface of the resist pattern and the ferromagnetic material layer, a depth of 20 nm is a limit for a ferromagnetic material to be etched. Thus, to carry out fabrication of a good aspect ratio, a thin resist cannot be used.
In the case of the high recording density HDD, a surface of the magnetic recording medium must be smooth to reduce spacing between the magnetic recording medium and the magnetic head. Accordingly, as shown in FIG. 1E, the nonmagnetic material layer 540 is buried in concave portions of the etched ferromagnetic material layer 520, and then the substrate surface must be smoothed in a CMP step. This CMP step imposes a load on the process for forming the patterned medium.
On the other hand, a medium of a discrete tracking system (IEEE Transactions on Magnetics Vol. 25, No. 5, P3381, 1989) has recently been proposed as one type of a patterned medium. This patterned medium has a magnetic layer formed only in a track region. The magnetic layer is formed in a region between tracks using ion milling or the like. However, there is a level difference of 20 to 50 nm attributable to presence/non-presence of a magnetic layer on the medium surface, and the level difference causes a problem of considerably reducing seeking durability.
In order to solve the problem of the level difference on the medium surface, a medium of a discrete system has been proposed, in which a magnetic layer that needs to become a region between tracks is made nonmagnetic by implanting nitrogen ions or oxygen ions therein (Japanese Patent Laid-Open No. 5-205257 (published in 1993)).
In addition, as a method for forming a patterned medium having a smoother surface, a method has been proposed for forming a patterned medium by selectively oxidizing a medium surface using a mask (U.S. Pat. No. 6,168,845).
In the above-described method of implanting oxygen ions or method of partially oxidizing the surface, since no etching steps are employed, the problem concerning the level difference on the surface due to ion milling does not occur. However, these methods cannot completely remove the level difference on the medium surface. This is because a volume of the oxidized region made nonmagnetic is increased, and the medium surface of the oxidized region is raised.
In the case of using oxidation reaction, since a mask material having high resistance to oxidation should preferably be used, a normal resist removing step such as an O2 ashing process cannot be used to remove the mask material. Consequently, the removing the mask material brings a process load.
Also, regarding the manufacturing of MRAM, necessary films including a lower ferromagnetic material layer, a tunnel oxide film layer, an upper ferromagnetic material layer, and the like are formed on a substrate, and then these layers are physically etched using ion milling when each memory element region is plotted. However, short-circuiting may occur between the upper and lower ferromagnetic material layers because of etching damage or etching residuals. Thus, it is desired to use a magnetic material patterning method having none of the above-described problems, high yields and good productivity.
On the other hand, the following problems exist concerning writing in the servo region formed in the magnetic recording medium. First, when a normal sample servo (sector servo) system is used, a step of writing servo information by using a conventional servo writer is necessary. Since head movement is controlled and the servo information is sequentially recorded in the respective servo regions of all tracks set on the magnetic recording medium, the step of writing servo tracking information is one of the steps that takes long time in the manufacturing process. In the future, a greater quantity of servo information will be necessary when a recording density is increased, requiring much longer time for the writing of the servo information by the servo writer. Thus, in order to mass-produce high recording density HDD devices inexpensively, it is required to shorten the time required for the step of writing servo information.
Furthermore, even in the case of the magnetic recording medium using the deep layer servo system, a step of forming a deep layer servo region is necessary in addition to the step of forming the magnetic recording medium. In the case of the deep layer servo system, especially, since writing of the servo information is carried out on the full surface, a time load for the writing is extremely large, and thus a request for shortening the time is stronger than that for the sample servo system.
Therefore, also for the writing of the servo information, instead of the method using the conventional servo track writer, it is desired to employ a magnetic material patterning method having high productivity and capable of writing servo information in the magnetic recording medium all at once.